This invention relates to a solid oxide fuel cell generator and, more particularly, to a generator having an array of closely spaced, tubular, axially elongated fuel cells for electrochemically reacting a fuel gas with an oxidizing gas.
Solid oxide fuel cell generators are employed to convert the chemical energy of a fuel gas such as natural gas into electrical energy. Typically, generators contain an array of literally hundreds or thousands of fuel cells which must be electrically connected in series and in parallel to produce the desired energy because each fuel cell develops a limited amount of energy at about 0.6 to 1.0 volt. For example, a state-of-the-art fuel cell having an active length of about 30-50 cm will produce approximately 20-40 watts. The fuel gas and an oxidizing gas, which is usually an oxygen-containing gas such as air, electrochemically react across the electrolyte boundary of the fuel cells at about 1000.degree. C. and produce carbon dioxide and water vapor.
State-of-the-art generators are disclosed by U.S. Pat. Nos. 4,898,792; 4,874,678; 4,728,584 and 4,395,468. These patents generally disclose solid oxide fuel cell generators having arrays of tubular, elongated fuel cells comprised of electrochemical cells supported on peripheral surfaces of hollow tubes which extend through a generator chamber. These generators generally employ designs wherein an oxidizing gas (typically air) is introduced into hollow support tubes via internal, concentric, injector tubes which extend at least about half the length of the support tubes The oxidizing gas flows toward the opposite ends of the hollow tubes, exits the injector tubes, reverses direction and flows back through the annulus between the support tubes and the injector tubes. This arrangement is designed to preheat the oxidant gas up to the reaction temperature and also to remove the excess heat generated by the electrochemical reaction in order to maintain the desired operating temperatures in the generator chamber. Typically (where the oxidant gas is air), up to 4-8 times the stoichiometrical amount of the oxidant gas needed to react with the fuel gas is fed into the tubes in order to maintain the nominal reaction temperature and the desired temperature profile in the generator.
Efficient operation of the prior art generators under low power, normal power and high power conditions is limited by many process variables, including the pressure drop of the gases flowing through the injector tubes and in the annular spaces around the injector tubes. The gas flow pressure drop through the tubes and the associated end effects tend to restrict high gas flow during high power conditions for a given generator design. In addition, temperature profiles across generators may vary because the fuel cells located at the peripheries of the generator housings tend to radiate substantial amounts of heat to the housings, which can be a substantial percentage of the excess heat generated, particularly during low electrical power operations. Typically (where air flows through the tubes of the fuel cells and fuel gases flow through the plena containing the fuel cells), the nominal reaction temperatures and thermal profiles are maintained by feeding a certain amount of excess oxidant gas to the interior fuel cells in an array and simultaneously feeding substantially less excess oxidant gas to the peripheral fuel cells because of the lower excess heat remaining after losing heat to the housings. Undesirably, this requires additional air controllers to regulate the air flow in the peripheral cells separately from the air flow to the interior cells.
Present state-of-the-art generators employ arrays of fuel cells having electrochemically active lengths of up to about 30-50 centimeters. The installation of hundreds and even thousands of long injector tubes in such fuel cells is both costly and very difficult to accomplish in commercial scale generators. Furthermore, the art anticipates that future generators will employ even longer fuel cells having active lengths of up to 100 centimeters or more in order to reduce the required number of fuel cells in a generator.
Accordingly, the art is searching for new generator designs which are less costly and difficult to assemble than are present designs, and yet are more efficient.